You have changed your base currency to Euros from what it was previously, eg Sterling.
You have made sure that the currencies of your various Bank, Charge, Supplier and Customer accounts are set correctly.
Now you need to make sure the asset and liability values brought forward are in Euros.
Click 
in the main toolbar.
Check that the 
display currency is set to Euro.
Click the Proposed B/Fwd values tab.
Click the 
button to reset the brought forward values. All values except for those Bank, Charge, Customer and Supplier accounts you have left in their original currencies (if any) will be in Euros.
Click 
to accept these values.